The invention relates to a method for adjusting the sulphur/sodium ratio in the flue gases of a soda recovery boiler when sulphurous odour gases are burned in the boiler.
Strong odour gases are produced in different steps of a sulphate pulp process, for instance in the digester house and the evaporation plant and during the stripping of condensates. In addition, strong odour gases are released from the expansion tank of the final concentrator. These gases contain various sulphurous compounds, e.g. H.sub.2 S, CH.sub.3 HS, (CH.sub.3).sub.2 S, (CH.sub.3).sub.2 S.sub.2. Diluted odour gases are released in the washer room and the tall oil cooking unit and together with vapours from dissolver tanks or the like. They contain small amounts of odorous sulphur such as H.sub.2 S or other odorous, mainly organic sulphur compounds. In this application and the appended claims the terms strong and diluted odour gases refer to the odour gases described above, whereas the term flue gases refers to gases which are contained in the soda recovery boiler and conducted to a flue gas duct and further onwards.
Odour gases are typically burned e.g. in a lime sludge reburning kiln or a separate odour gas boiler to recover their heat content and to separate sulphur from the flue gases so that it can be recycled to the chemical cycle. If odour gases with a high sulphur content are supplied to a soda recovery boiler, the amount of sulphur--typically sulphur dioxide and to some extent sulphur trioxide--increases in the flue gases. The excessive presence of these compounds renders the fly ash acidic, which results in the clogging of the heat surfaces of the boiler and, without a separate flue gas scrubber, even in an increase in sulphur emissions.
The building of a separate odour gas boiler or odour gas burning apparatus and its use in the removal of odour gases requires major investments and, in addition to odour gases, fuel must continuously be fed into it in order to keep the burning under control. Moreover, the resulting flue gases must be scrubbed, which in turn causes extra operating costs.